Download Describing Chemical Reactions

Describing Chemical
Reactions
Objectives:
List three observations that suggest that a chemical reaction has
taken place
List three requirements for a correctly written chemical equation
Write a word equation and a formula equation for a given
chemical reaction
Balance a formula equation by inspection
What Is a chemical reaction?
 Chemical reaction: the process by which one or more
substances are changed into one or more new substances
 Represented by chemical equations
 Chemical equation: a shorthand expression that
represents a chemical reaction
 Shows the relative amount of each substance taking
place in a chemical reaction
Signs of a Chemical Reaction
 To know for certain that a chemical reaction has taken
place requires evidence that one or more substances have
undergone a change in identity
 Certain easily observed changes usually indicate that a
chemical reaction has occurred:
 Evolution of heat and light
 Production of a gas
 Formation of a precipitate
 Precipitate: a solid that is produced as a result of a chemical
reaction in solution and that separates from the solution
 Color change
Chemical Equations
A+B  C+D
REACTANTS
PRODUCTS
Reactants: starting
substances in a chemical
reaction
Products: the
substances formed in
a chemical reaction
Characteristics of a Chemical
Equation
 The equation must represent known facts.
 All reactants and products must be identified.
 The equation must contain the correct formulas for the
reactants and products.
 The law of conservation of mass must be satisfied
Law of Conservation of Mass
 Mass is neither created nor destroyed in a chemical
reaction
 Total mass stays the same
 Atoms can only rearrange
Writing Equations
 Identify the substances involved
 Use symbols to show:
 How many? – coefficient
 Coefficient: a small whole number that appears in front of a
chemical formula in a chemical equation
 Of what? – chemical formulas
 In what state? – physical state
 Letters in parentheses indicate the physical state of each
substance involved in the reaction
 (g) gas ; (l) liquid ; (s) solid ; (aq) aqueous solution
Balancing Chemical Equations
1. Write the unbalanced chemical equation.
2. Count the atoms of each type on each side of the equation.
3. Add coefficients to make the number of atoms equal.
(Remember: the law of conservation of mass must be
satisfied!)
Coefficient subscript = # of atoms
4. Reduce coefficients to the lowest possible ratio, if
necessary.
5. Double check atom balance!
Sample Problem
 Aluminum, Al, and copper (II) chloride, CuCl2, react to
form copper, Cu, and aluminum chloride, AlCl.
CuCl2 
Al +
2
Cu +
3
AlCl3
3
2
2
2
1
Al
1
3
1
Cu
1
3
6
2
Cl
3
6
Helpful Tips for Balancing Equations
 Balance one element at a time
 Update ALL atoms counts after adding a coefficient
 If an element appears more than once per side, balance it last
 Balance polyatomic ions as single units
 Ex: “1 SO4” instead of “1 S” and “4 O”
 If you find an element difficult to balance, save it for last
 If you use a fraction during the balancing process, you can
eliminate it later by multiplying each coefficient by 2
 In general, it may be helpful to balance H and O last
Chagi Fan!
Balance each of the following chemical reactions:
1. Mg(s) + O2(g)  MgO(s)
2Mg + O2  2MgO
2. Fe(s) + O2(g)  Fe2O3(s)
4Fe + 3O2  2Fe2O3
Practice
 Balance the following equations:
1.
2.
3.
4.
5.
6.
__Mg + __HCl  __MgCl2 + __H2
__H2 + __Cl2  __HCl
__ Ga + __ O2  __ Ga2O3
__Al + __Fe2O3  __Al2O3 + __Fe
__GeCl4 + __ H2O  __ GeO2 + __ HCl
__Fe + __H2O  __Fe3O4 + __H2